So I did some more tests on various circuits with the recent Group-C cars and the Jaguar XJR-9 is definitely and without a doubt the fastest of them all. On all tracks that take about 1:30-1:45 to lap the XJR-9 was 1.5-1.8 seconds faster for me, the slowest of them all is, expectedly, the Mazda 787B, but only marginally, around a quarter of a second slower than the times of the R92CP and the Sauber C9.
But the Mazda is extremely well-mannered, no Turbo that kicks like crazy, no tendencies for oversteering, very predictable handling. It might be a good choice for endurance races with changing weather.
The Jaguar XJ-9 definitely needs LSD dialed up from 40 to 30 and rear camber decreased to 1.0 and maybe even 0.7 in order to tame the rear end on low speeds. Increasing rear down force does not help much since the oversteering occurs at low speeds. You'd have to increase rear down force massively, seriously decerasing high speed cornering.
Just my 2 cents.